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<h2>NUnit Addins</h2>

<div style="text-align: center; margin: 2em 10%; padding: 4px 0; border: 2px solid black">
  <h4>Preliminary documentation, subject to change.</h4>
</div>

<p>NUnit originally identified tests in the time-honored way used in many xUnit
test frameworks. Test classes inherited from the framework's 
TestCase class. Individual test case methods were identified by having names
starting with "test..."</p>

<p>With NUnit 2.0, we introduced the use of attributes to identify both fixtures
and test cases. Use of attributes in this way was a natural outcome of their 
presence in .NET and gave us a way of identifying tests that was completely
independent of both inheritance and naming conventions.</p>

<p>However, by moving away from an inheritance-based mechanism we no longer
had an easy way for others to extend NUnit's internal behavior. NUnit Addins are 
intended to fill that gap, providing an mechanism to introduce new or changed 
behavior without modifying NUnit itself.</p>

<h3>Extension Points, Extensions and Addins</h3>

<p>NUnit provides a number of <b>Extension Points</b>, places where it is
possible to extend its behavior. Because NUnit works with various hosts
and uses separate AppDomains to run tests, <b>Extension Points</b> are 
categorized into three types: Core, Client and Gui. Each of these types is
supported by a different <b>Extension Host</b>.

<p>An NUnit <b>Addin</b> provides enhanced functionality through one or more 
extensions, which it installs at identified <b>Extension Points</b>. Each
<b>Addin</b> is characterized by the types of extensions it supplies, so that
an <b>Extension Host</b> knows whether to invoke it.</p>

<blockquote>
<p><b>Note:</b> In the current release, only Core extensions are actually
supported. An Addin may characterize itself as a Client or Gui extension and
will be listed as such in the <a href="addinsDialog.html">Addins Dialog</a>, 
but no other action is taken.</p>
</blockquote> 

<h3>Addin Identification, Loading and Installation</h3>

<p>NUnit examines all assemblies in the <b>bin/addins</b> directory, looking
for public classes with the <b>NUnitAddinAttribute</b> and implementing the
<b>IAddin</b> interface. It loads all those it finds as Addins.</p>

<p><b>NUnitAddinAttribute</b> supports three optional named parameters: Type,
Name and Description. Name and Description are strings representing the name
of the extension and a description of what it does. If no name is provided,
the name of the class is used. Type may be any one or a combination of the 
ExtensionType enum members:

<pre>
	[Flags]
	public enum ExtensionType
	{
		Core=1,
		Client=2,
		Gui=4
	}
</pre>

The values may be or'ed together, allowing for future Addins that require
extensions at multiple levels of the application. If not provided, Type
defaults to ExtensionType.Core.</p>

<p>The <b>IAddin</b> interface, which must be implemented by each Addin,
is defined as follows:</p>

<pre>
	public interface IAddin
	{
		bool Install( IExtensionHost host );
	}
</pre>

<p> The <b>Install</b> method is called by each host for which the addin has
specified an ExtensionType. The addin should check that the necessary extension
points are available and install itself, returning true for success or false
for failure to install. The method will be called once for each extension
host and - for Core extensions - each time a new test domain is loaded.</p>

<p>The Install method uses the <b>IExtensionHost</b> interface to locate
extension points. It is defined as follows:</p>

<pre>
	public interface IExtensionHost
	{
	 	IExtensionPoint[] ExtensionPoints { get; }
		IFrameworkRegistry FrameworkRegistry{ get; }
		IExtensionPoint GetExtensionPoint( string name );
	}
</pre>

<p>The <b>ExtensionPoints</b> property returns an array of all extension points
for those extensions that need the information. The <b>FrameworkRegistry</b>
is provided for advanced extensions that emulate external test frameworks. See
the source code for details.</p>

<p>Most addins will only need to use the <b>GetExtensionPoint</b> method to
get the interface to a particular extension point. The <b>IExtensionPoint</b>
interface is defined as follows:</p>

<pre>
	public interface IExtensionPoint
	{
		string Name { get; }
		IExtensionHost Host { get; }
		void Install( object extension );
		void Remove( object extension );
	}
</pre>

<p>Once again, most addins will only need to use one method - the
<b>Install</b> method in this case. This method passes an extension object
to the <b>Extension Point</b> where it is installed. Generally, extensions
do not need to remove themselves once installed, but the method is
provided in any case.</p>

<h3>Extension Point Details</h3>

<p>Depending on the particular extension point, the object passed will
need to implement one or more interfaces. The following <b>ExtensionPoints</b>
are implemented in this release of NUnit:

<ul>
	<li><a href="#suiteBuilders">SuiteBuilders</a>
	<li><a href="#testBuilders">TestCaseBuilders</a>
	<li><a href="#testDecorators">TestDecorators</a>
	<li><a href="#eventListeners">Listeners</a>
</ul></p>

<p>For examples of implementing each type of extension, see the Extensibility
samples provided with NUnit. More complete examples are available in the
code of NUnit itself, since NUnit uses the same mechanism internally.</p>

<h4><a name="suiteBuilders">SuiteBuilders</a></h4>
<p>
<p>Addins use the host to access this extension point by name:

<pre>
	IExtensionPoint suiteBuilders = host.GetExtensionPoint( "SuiteBuilders" );</pre>

<p>The extension object passed to Install must implement the ISuiteBuilder interface:

<pre>
	public interface ISuiteBuilder
	{
		bool CanBuildFrom( Type type );
		Test BuildFrom( Type type );
	}
</pre>

<p>The BuildFrom method should return a test fixture completely populated
with its contained test cases.

<h4><a name="testBuilders">TestCaseBuilders</a></h4>
<p>
<p>Addins use the host to access this extension point by name:

<pre>
	IExtensionPoint testCaseBuilders = host.GetExtensionPoint( "TestCaseBuilders" );</pre>

<p>The extension object passed to Install must implement the ITestCaseBuilder interface:

<pre>
	public interface ITestCaseBuilder
	{
		bool CanBuildFrom( MethodInfo method );
		Test BuildFrom( MethodInfo method );
	}
</pre>

<p>Note that this extension point will be called for methods in any type
of fixture. If the addin is intended to only work on methods within
a particular type of fixture, the CanBuildFrom method must check
the fixture type.

<h4><a name="testDecorators">TestDecorators</a></h4>
<p>
<p>Addins use the host to access this extension point by name:

<pre>
	IExtensionPoint testDecorators = host.GetExtensionPoint( "TestDecorators" );</pre>

<p>The extension object passed to Install must implement the ITestDecorator interface:

<pre>
	public interface ITestDecorator
	{
		Test Decorate( Test test, MemberInfo member );
	}
</pre>

<p>The Decorator may do several things, depending on what it needs
to accomplish:
<ol>
  <li>Return test unmodified
  <li>Modify properties of the test object and return it
  <li>Replace test with another object, either discarding the
  original or aggregating it in the new test.
</ol>

<h4><a name="eventListeners">EventListeners</a></h4>
<p>
<p>Addins use the host to access this extension point by name:

<pre>
	IExtensionPoint listeners = host.GetExtensionPoint( "EventListeners" );</pre>

<p>The extension object passed to Install must implement the EventListener interface:

<pre>
	public interface EventListener
	{
		void RunStarted( string name, int testCount );
		void RunFinished( TestResult result );
		void RunFinished( Exception exception );
		void TestStarted(TestName testName);
		void TestFinished(TestCaseResult result);
		void SuiteStarted(TestName testName);
		void SuiteFinished(TestSuiteResult result);
		void UnhandledException( Exception exception );
		void TestOutput(TestOutput testOutput);
	}
</pre>

<p>You must provide all the methods, but the body may be empty for any
that you have no need of.

<h3>Tips for Writing Extensions</h3>

<p>An Extenders Guide will be published in the future. At the moment, writing an
extension is a bit of an adventure. Extension authors are advised to join the 
nunit-developer list and post questions and comments there.

<p>For the moment, the following tips may be of assistance.
<ul>
<li>The <b>nunit.core.interfaces</b> assembly is intended to be stable in the future
while the <b>nunit.core</b> assembly will change from release to release. Right now,
both assemblies are still in flux, but extensions that depend solely on the interfaces
assembly will have a much better chance of surviving NUnit version changes. Unfortunately,
this is rather difficult to do without duplicating a great deal of NUnit code. Most
of the add-in samples provided with NUnit are currently version dependent.

<li>If you place the definition of a custom attribute in the same assembly as your
add-in, then user tests are dependent on the add-in assembly. If the add-in is 
version-dependent, then the user tests will also be version-dependent. We suggest
placing any types referenced by user tests in a separate assembly, particularly if
your extension relies on nunit.core.

<li>If using Visual Studio, set Copy Local to false for any references to nunit.core
or nunit.core.interfaces. This is especially important if you are also building
NUnit itself.

<li>There is currently no mechanism to allow decorators to apply in a particular order.
NUnit applies decorators in the order in which they are returned through reflection,
which may vary among different runtimes.

<li>Avoid trying to "stretch" the existing extension points to do more than they were
intended to do. Rather, let us know what further extension points you would like to see!
</ul>


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